Discharge tube



Patented Aug. 18, 1936 UNITED STATES PATENT OFFICE DISCHARGE TUBE Austria Application June 8, 1932, Serial No. 616,014 In Austria July 8, 1931 c claims.

This invention relates to discharge tubes and has for its object improvements over tubes disclosed in our copending application Ser. No. 492,020 of October 29, 1930, in which indirectly heated hot cathodes for discharge tubes are described, the heater of which can be operated at fuel line voltages or at voltages which are high compared with the usual range of voltages employed at the present time, namely 4-6 volts for alternating current or from 4 to 15 or even 20 volts for direct current. This heater comprises a very long and thin spiralized or coiled wire, which is accommodated in at least two helix sections led to and fro in an insulating material surrounded by a relatively small cathode, so that the cathode may be brought to the emission temperature with a very small energy input.

Now according to one part of the present invention only that portion of the spiralized or coiled heating wire which is disposed within the insulating body, has the shape of a helix while the connecting ends which lie outside this insulating body and which are intended for attachment to the lead-in poles are straight. This precaution obviates the danger that the connecting ends which are not surrounded by insulating material, and are therefore, exposed, and which moreover lie in very close proximity to each other, may become too hot, and short-circuiting may occur between them. It will be noted that it is necessary to arrange the connecting ends in very close proximity to each other, since to enable the working of the cathode with the above mentioned very small energy input this cathode and therefore also the insulating body encased by the cathode must have the smallest possible diameter. With an insulating body of such small dimensions it is self-evident that the connecting ends of the heating wire, when these are led out at the same side of the insulating body, cannot have more than a very little distance from each other. If alternating current is used the close proximity of the connecting -ends of the heating wire is not a disadvantage but on the contrary is very desirable since on the one hand the mutual inductive inuences of the alternating current in the connecting wires are considerably reduced and on the other hand the desirable shielding of the said connecting ends carrying the alternating current from the other electrodes may be achieved in a very simple manner. The danger of short-circuiting due to the near proximity of the connecting ends of the heating lilament arises, for example, when the cathode is freed of gas during the manufacturing of the (Cl. Z50-27.5)

tubes while the incandescent filament is being heated, since in this case high voltage is applied to the incandescent filament, while at the same time large quantities of gas escape from the cathode. 'Ihese quantities of gas become ionized, 5' and when the heating wire leads-in situated in this ionized gas atmosphere attain a high temperature, a discharge can take place between them which causes short-circuiting. If the ends of the heating wire leading from the connecting l0` poles to the cathode body are coiled, these ends become very hot, due to the mutual heating up of the very closely juxtaposed helical turns, and the ydangerous discharge just mentioned can take place. On the other hand if the arrangement l5 described in the present application is used, that is to say when the connecting ends of the heating wire are of straight form, the temperature of these ends remains considerably lower tor the same current, and in particular remains too low 20 to give rise to a discharge across the ionized gas gap.

With the construction of cathode described in our copending application Ser. No. 492,020 not merely the connecting ends of the heating wire 25 but also those portions of the heating wire which constitute the connection between every two sections of the heating wire encased by the insulating body are exposed. It can be desirable to make these connecting pieces straight also, again 3o with a view of keeping the temperature of these connecting pieces as low as possible. The portions of the heating wire not encased in insulating material have a greater tendency to expand than the portions enclosed by the insulating body, 35 so that, in view of the narrowness of the available space, there is also a danger of short-circuiting at the reversing points of the zig-Zagged heating wir-e, that is to say at the connecting pieces between the individual sections of the heat- 40 ing wire encased by the insulating body.

The invention further consists in eliminating altogether such connecting portions of the heating wire as project beyond the insulating body, by surrounding the heating wire, with the excep- 45 tion of its connecting ends, but including its connecting pieces between the individual sections of the heating wire, with insulating material. This is accomplished by providing depressions in the ends of the insulating body the con- 50 necting pieces between the individual sections of the heating wire being located in said depressions.

It is to be noted that the uncommonly line and very long coiled wire, which is used as the 55 heater filament and which is accommodated freely in bores in one or more insulating bodies possesses practically no inherent stiiness, so that it can lose its shape very easily, particularly under the eiect of its own weight. For example, a helix of this nature, when disposed vertically, and in the absence of special precautions, would collapse, that is to say the upper portion of the helix would become stretched, Y

while the lower turns would become closely crowded upon each other. A further object-of this invention is to eliminate the possibility of such alteration in shape` This is accomplished by supporting the long coiled heating wire at as large a number of points in its length as possible.

The supporting of the heating wire can be accomplished in a particularly simple manner;

that is to say without the use of any supplementary supporting members by disposing the cathode horizontally. In this case the heating wire helix (or the section of the heating wire helix concerned) disposed in the lying position in the bore of the insulating body is supported by the insulating body itself at all those points at which the turns of the helix bear upon the inner surface of the bore.

A supporting of the heating wire which is `not so perfect as the above described but which in many cases is still sunicient may be obtained by a moderately inclined arrangement of the cathode.

In the drawing Fig. 1 shows, on a very much enlarged scale, a plan view'of the cathode, Fig. 2 a section on the line A-B, and Fig. 3 a section on the line C-D in Fig. 1. The horizontally arranged cathode I which carries the usual emitting layer surrounds an insulating body 2 having, for example, four bores 3, 4, 5 and I5 through which the very thin heatingwire of a diameter' materially less than 0.1 mm, i. e. of 0.02 mm, I is conducted to and fro. As stated in said copending application, the internal diameter of the cathode tube I Vis approximately 3 mm. The connecting ends 8 and 9 of this heating wire are straight as distinct from the remaining coiled portion of the heating wire. The insulating body 2 is provided at its upper end with two depressions I0 and II, and at its lower end with a depression I2. A connecting piece I3 isY placed in the depression I0 between the sections I4 and I5 of the heating wire laid in the bores 5 and while in the depressions II and I2 there are fitted the conncting pieces between the sections of the heating wire taken through the bores 3 and 4, and 3 and 5, respectively. The depressions I0, I I and I2 are of such depth (for example about 2 mm.) that the connecting pieces between the individual sections of the heating wire can freely expand therein when heated, without projecting beyond the insulating body, since in the latter event they could come in contact with each other and thus cause short-circuiting. The said connecting pieces can also be made straight in the above described manner, instead of being coiled, although in general there willV be no necessity for this straight shaping of the connecting pieces when these connecting pieces'are also arranged within Ythe insulating body in the manner shown,

If desired, the insulating body provided with the described depressions can further be entirely closedV at one or at both ends, for which purposeeither thin end walls, closure caps, or the like can be provided.

The straight connecting ends of the heating wire can be obtained either by stretching the ends straight, after the coiling of the wire but before the introduction of the same into the cathode, or it is possible to arrange for the ends and, if desired, also those portions of the heating wire which are destined to form the connections between the individual sections of the heating wire, to remain straight during the coiling process.

What we claim, is:

1 'In an electric discharge tube, an indirectly heated cathode adapted to be heated by current supplied at high voltages, such as the line voltage, said cathode comprising a single coherent insulating body having a. plurality of longitudinal bores and transverse grooves interconnecting said bores, the diameter of said insulating body being approximately 3 mm., an emitting layer surrounding said body, and a heating wire having a diameter substantially less than 0.1 mm. and being of such length that when a high voltage is applied to said wire its temperature remains substantially equal to that of the emitting layer, said wire having portions wound in the form of a helix and situated within said bores, the portions of the wire interconnecting the helical portions being situated within said grooves.

2. In a cathode for electrical discharge tubes, a cylinder of insulating material having projections and a plurality of spaced longitudinal bores, a heating wire wound into a helix except its ends which are straight and passed back and fourth through said bores so that the wire coils contact substantially throughout with the insulation surrounding the bores except the helix portions connecting from one bore to the other which lie against the ends of the cylinder, said ends being so shaped that said helix portions connecting from one bore to the other are free to expand in the direction of the axis of the cylinder but their sides are confined within the projections of said insulating material which are high enough to guard against contact of adjacent heating wire portions even when the latter is fully expanded, the two straight ends projecting outside of the cylinder from two bores at one end of the cylinder, and an emitting layer covering the entire outside surface of said cylinder.

3. In the structure defined in claim 2, a closure of insulating material for at least one end of said cylinder.

4. In a cathode for electrical discharge tubes, a cylinder of insulating material having a plurality of spaced longitudinal bores, a heating wire wound into a helix except at its ends which are straight and passed back and forth through said bores so that the wire coils contact substantially throughout with the insulation surrounding the bores, the helix portions connecting from one bore to the other lying entirely within the Vends of the cylinder, the two straight ends projecting outside of the cylinder from two bores at one end of the cylinder, and an emitting layer on the outside surface of said cylinder.

5. In a cathode for electrical discharge tubes, a cylinder of insulating material having .a plurality of substantially uniformly spaced longitudinal bores, a heating wire wound into a helix except its ends which are straight and passed back and forth through said bores so that the wire coils contact substantially throughout with the insulation surrounding the bores, the helix portions connecting from one bore to the other lying en- 

